Why do astronomers believe that galactic collisions cause active galactic nuclei?
What will be an ideal response?
All galaxies are believed to contain supermassive central black holes. Active galactic nuclei are caused when a large amount of matter falls into the central black hole, triggering an eruption. During a galactic collision, galaxies become tidally distorted, sending excess matter into the central black hole.
Observations support this idea. Many active galaxies have distorted shapes. Over a third of active galaxies are found to be double objects where the active nucleus is orbiting an object of similar mass. Active galaxies are more common in the early Universe, when galactic collisions were more frequent.
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Two particles whose charges have equal magnitudes q but opposite signs are placed as shown. The reference point for zero potential is infinity. 
At which labeled point is the potential largest?
A. A B. B C. C D. D E. E F. None of the labeled points
An air-filled 3.0 nF capacitor is charged to 8.0 V. If the plate separation is 100
m what is the energy density in the electric field?
a. 
b. 
c. 
d. 
e. 
Potential of Point-Charges: Four charged particles (two having a charge +Q and two having a charge -Q) are arranged in the xy-plane, as shown in the figure. These particles are all equidistant from the origin. The electric potential (relative to infinity) at point P on the z-axis due to these particles, is
A. zero. B. positive. C. negative. D. impossible to determine based on the information given.
A proton is accelerated from rest through a potential difference of 150 V. It then enters a region of uniform magnetic field and moves in a circular path (radius = 12 cm). What is the magnitude of the magnetic field in this region?
A. 18 mT B. 12 mT C. 15 mT D. 22 mT E. 10 mT